Automatic tuning control



Jan. 25, 1944. c. w. HANSELL 2,339,851

AUTOMATIC TUNING CONTROL Fil d May 28, 1941 v Fig 1 OUTPUT -10 /2 l4 /6 3 S/G/VAL usrmoam; AMPLITUDE FREQUENCY f osrscroe & UM/TER& SHIFT [Gym AMPLIFIERS AMPLIFIER DETECTOR g2 /5 16' i; 1: l-IETERODYNE 20 OSCILLATOR- w v 8 msousn/cr FREQUENCY CONTROLLED SELECT/VF 24 322mm may 400/0 5mm FREQUENCY O OSCILL/UUR J? u @jza RECTIFIER 3s 39 ilsmj L? "3 1ml m 4 28 30 V 34 gm 66666 628W FREQUENCY 'NVENTOR c. w. HANSELL BY 4' ATTORNEY shift.

Patented Jan. 25, 1944 AUTOMATIC TUNING CONTROL Clarence W. Hansell, Port Jefferson, N. Y., as

signor to Radio Corporation of America, a corporation of Delaware Application May 28, 1941, Serial No. 395,556

16 Claims.

v This application concerns a new and improved automatic tuning control fora receiver used to receive signals sent by transmitter frequency In 'my United States application Serial No. 324,051, filed March 15, 1940, now U. S. Patent No.- 2,293,501, dated August 18, 1942, I have pointed out some of the advantages of signalling by means of frequency shift at the transmitter as compared with the usual method of signalling in which the transmitter power is keyed on and off. Outstanding advantages of frequency shift keying are that it permits use of amplitude limiting at the receiver and makes it possible to greatly reduce the signal distorting effects of signals arriving at the receiver over multiple paths of different time delay. Other advantages are that the average transmitter power is increased, be-

cause we have the equivalent of two transmitters carrying the same message, and the problem of reducing undesired transmitter side band frequency power is reduced.

In my United States application Serial No. 381,945, filed March 6, 1941, I have disclosed a receiver for use with frequency shift transmission or other frequency varied transmission. An advantage of this receiver is the means therewith for improving the signal-to-noise ratio of the modulation output. In this receiver two paths are provided to pass freely frequencies of the shift frequencies and groups of frequencies adjacent thereto and to reject other frequencies and noise components which fall outside of the frequency spectrum passed freely.

In my application Serial No. 366,860, filed November 23, 1940, Patent No. 2,299,388, October 20, 1942, I have disclosed means for improving frequency shift communication by causing momentary overswing of frequency shift at the transmitter, equivalent to preemphasizing higher No. 1,803,504 I have described diversity receiving systems for the reception of signals sent by phase or frequency shift, or modulation, of a carrier wave current.

The present invention constitutes an improve- Iment upon, and an addition to, these previously applied for inventions and may be used in combination with them as well as in combination with other inventions made by myself and others.

. One of the problems associated with frequency shift keying and with phase or frequency modulation generally is to provide for automatic tuning at. the. receiver so that the receiver. will re a frequency shift keyed transmitter.

main in optimum adjustment and so that the frequency selectivity of the receiver need not be sacrificed to allow for relative frequency drifts between receiver and transmitter.

It will be evident that automatic tuning cannot be'carried out satisfactorily, in rec'eivingfrequency shift telegraph, or similar kinds of transmission, by the usual methods applied to amplitude modulated constant carrier-frequency transmission. ,If the ordinary methods were used then the automatic tuning would tend to follow the transmitter keying and would make undesired changes in receiver adjustment whenever long dashes or long spaces wereheld at the transmitter. Likewise, in facsimile or television trans-- mission, where frequency shift should follow variations in light intensity theusual type of automatic tuning system would tend to eliminate responses to background light, which require direct current, or extremely low frequency'responses. In consequence, receiver selectivity would have to be sacrified.

The scheme here described provides means to keep receivers in substantially perfect adjust ment without any tendency for falsetuning adjustments to takeplaceas a result of useful steady'state frequency shifts such-as would-be produced by long dashes or long spaces held at In the present system, correction takes place only while the transmitter is being keyed or modulated and cannot be biased in adjustment due to preponderance oftime given to marking or spacing.

At the same time there is no theoretical limitation to the range of frequency in which the automatic tuning system will function. In other words, my system does not require the transmitter and receiver to have nearly correct reladetail, reference will be madejto the attached drawing wherein: h p 1 s Figure 1 illustrates the essential elements of a spaced wave receiver with automatic frequency controlmeans arranged in accordance with my invention; while 7 Figure 2 is a curve used to illustrate the operation of my system.

In Figure 1, I have showna receiving system comprising an antenna l0 which picks up wave energy comprising'a first'frequency which may be referred to as the spacing frequency and the side bands thereabout resulting from modulation of keying and a second frequency which may be called the marking frequency and the side bands thereabout resulting from modulation or keying. The antenna delivers this received frequency shift keyed radio frequency power to detector and amplifier I2 wherein itis heterodyned:

with oscillatory energy from 8 and the amplified intermediate frequency currents are delivered from [2 to an amplitude modulation suppressor or limiter and amplifier M which removes variations in amplitude from th signal currents.

From the amplitude limiter i4 current is deliv-- ered to a frequency shift detector l6 from which is provided potential and current which changes in strength, and/or in direction, in response to signal frequency shifts. The output curren't may be recorded or put to other use as desired. For example, I have shown means to use this current to control a tone keyer l8 to turn on and off an audio frequency current supplied from source 20 by means of which the signals may be transmitted over a wire line multiplex carrier current channel 22 to a central traffic ofiice where the messages may be first recorded as :an undulant line on paper tape, and then transcribed on message blanks.

To provide forautomatic tuning control of the receiver I take an adjustableportion of the amplitude limited intermediate frequency current from unit 14 and pass it'through a frequency selective circuit 24 and rectifier 26, the selective circuit having a frequency response such as that illustrated qualitatively in Figure 2. The resistances provide an approved means for regulating the amplitude'of the current supplied to the selective circuit 24. From the frequency selective circuit the current is delivered to the rectifier 25 and from the said rectifier to the pri mary windings PI and P2 of two transformers ZB -and 30. When the transmitter is keyed the two transformers 2B and 30 deliver pulses to two rectifier systems 34 and 36. The pulses delivered to the rect fiers 34 and 36 reverse in polarity as the intermediate frequency varies above or below a correct value with respect to the center frequency 0-! (Figure 2) of the frequency selective circuit 24-. The manner in which these pulses are produced and their polarity reversed as the intermediate fre uency varies above or below a correct value with respect to the center frequency Ci of frequency selective circuit 24 will be understood from the following description thereof madewith reference to Figures 1 and 2. A change in the frequency of the keyed wave from C to D in Figure 2 is accompanied byan increase in amplitude of the vol age applied through selective circuit 24 to rectifier 26. The current output of rectifier 26 increases, This changing current flowing through the primary windings PI and P2 develops a voltage of a certain sign in the secondary windings during the period of change.

velops a voltage of a sign opposed to said certain sign in the secondary windings of transformers 28 and 30 during the period of change.

These pulses, the polarity of which reverse, and the currents of reversible polarity are produced-only from th modulations when the carrier has deviated from its normal value. This will be seen by reference to Figure 2, wherein if the carrier is proper, a change from space frequency to mark frequency. that is, between points A and B, is notaccompanied ,by a change in amrespect to each other.

plitude of the voltage applied to 26 or a change in the current intensity out of 26.

At the same time a controlled portion of the receiver, output current is fed by adjustable resistances 38 in lines 39 to the primary windings P3 and P4 of the transformers 28 and and thence to the two rectifier systems 34 and 36. However, the polarities of these pulses to the two rectifier transformers 28 and 30 are reversed with This reversal of polarity of the pulses is attained by proper connections between lines 39 and the windings P3 and P4. The rectifiers RI and R2 are connected as shown in opposed relation with respect to the transformer 28 secondary winding. The rectifiers R3 and R4 are connected in a like manner to the secondary winding of transformer 30. Potentials which are in series but opposed are produced in the resistances 40 and 42 which are shunted for frequencies of modulation frequency by condensers 44 and 46.

In operation, when the received carrier current frequency is being modulated, or keyed between two values, the two rectifier systems 34 and 3G deliver equal and opposite direct current output potentials, which substantially compensate, to lines 56 whenever the receiver is in correct adjustment, that is, when the intermediate frequency center frequency is at, say, CI (Figure 2). If the adjustment changes so that the intermediate frequency currents are not at the correct frequencies then one rectifier or the other provides a larger current and the other provides a weaker current. The over-all result is that the two rectifiers in series deliver a resultant direct current potential and current which reverses in polarity and changes in amplitude according to the direction and amount of the frequency error in intermediate frequency current. The resultant current is then supplied by lines to the means in unit 8 and used to correct the intermediate frequency by automatically adjusting the frequency supplied by the generating means 8 to the detector 12.

In correcting the oscillator frequency I prefer to employ a reverse current relay controlled by the resultant current from the two rectifiers and this relay will, in turn, start a small motor, in one direction or the other, and stop the same when the adjustment is proper, to control a small variable condenser in the circuits of the wave generator 8. Preferred forms of control and tuning means for use here are shown in my United States Patents No. 2,095,980, dated October 19, 1937, and No. 2,104,801, dated January 11, 1938, and in my United States applications Serial No. 274,601, filed May 19, 1939, now United States Patent No. 2,243,702, dated May 27, 1941, and Serial No. 358,775, filed September 28, 1940, now U. S. Patent No. 2,290,327, dated July 21, 1942.

Referring to Figure 2, if the intermediate frequency current is correct and is keyed between spacing frequency having a value A and marking frequency having a value B then for each frequency value there is delivered equal currents from the frequency selective circuit 24 to the rectifier 26 and as a result substantially no pulse or alternating current component of current is fed from rectifier 26 to transformers 28 and 30 to induce currents in the secondaries thereof. The output of It does supply pulses to the transformers 28 and 30 but these pulses are equal in both transformers and induce equal currents in the secondary windings which oppose to balance out after passing through rectifiers 34 and 36 to 2,889,851 line 50. 'I'Iowever, if the intermediate frequency current decreases in frequency to values C and D the currents delivered to the rectifier 26 for each frequency value are unequal and pulses of a certain polarity are delivered by rectifier 26 1 to windings PI and P2. The combined currents from detector 16 and rectifier 2B delivered to the rectifier systems 34 and 36 are unequal and a resultant potential is developed across 40 and 42 in series the polarity of which depends on the polarity of the pulse supplied by rectifier 26 to windings PI and P2. The differential potential developed across 40 and 42 is used via lines 50 to cause correction in frequency. If the intermediate frequency increases to values E and F pulses will be delivered from rectifier 26 to the transformers 28 and 40 and thence to the differential rectifiers butthey will be reversed in polarity, with respect to the pulses delivered by rectifier 26 when the intermediate frequency is at C and D, and so will'reverse the unbalance between the differential rectifier output currents and bring about a correction in the required direction.

Thus by providing one set of pulses, or modulations, representative of the signal and another J set of pulses derived from the frequency selective circuit 24, in response to the signals, having a polarity which reverses as the receiver tuning passes through a correct value, I am able to derive a current which reverses polarity when the tuning passes through the correct value, and which increases in strength as the tuning departs further from a correct adjustment, in either direction.

In cases where multipath effects are important,

as for long distance radio transmission viathe ionosphere, I prefer to combine two or more of these receivers into a diversity receiving system such as, for example, that described in my United States application Serial No. 326,129, filed March 27 1940, now U. S. Patent No. 2,249,425, dated July 15, 1941, and in my Patent No. 1,803,504.

The automatic tuning control system described is not limited in application to receivers used with spacing wave or frequency shift keyed signals. It is also directly applicable to reception of ordinary telephone type of frequency or phase modulation transmissions of either the narrow or wide band varieties.

In carrying out the invention I prefer to employ a vacuum tube rectifier at 26, after the frequency selective circuit for converting intermediate frequency currents into pulses in response to frequency shifts but the other rectifiers, not including the detectors in unit 16, may be of the small dry disc types often used in rectifier type alternating current measuring instruments. These are very small and inexpensive and require no cathode heating power. Consequently the cost of applying the system I propose is reasonable.

I claim:

1. In a system for detecting changes in the mean or average frequency of a modulated alternating current, means to demodulate the current,

means to derive another modulation frequency current which reverses in polarity as the mean or average frequency passes through an arbitrary value, means to combine the demodulated and 3.. In an automatic tuning system for phase or frequency modulated carrier wave receivers, a demodulator to reproduce modulating currents, another demodulator with a frequency discriminator circuit tuned for maximum amplitude at the mean or carrier wave frequency, a circuit for combining and rectifying output currents from the two demodulators to obtain a direct current and control elements responsive to the direct current for controlling the mean or carrier wave frequency of the modulated carrier wave.

4. Means for indicating the relation between the frequency of a timing modulated carrier cur rent and the resonant frequency of a circuit comprising, means to derive modulation frequency currents corresponding to timing modulations of the carrier current, means to derive modulation frequency currents which reverse polarity as the carrier current frequency moves through the resonant frequency, and means to derive a direct current from the two modulation frequency currents the polarity and magnitude of which is an indication of relative frequency.

5. An automatic receiver tuning control system responsive only While a received current is being modulated comprising, means to derive from the modulations two alternating currents, one of which reverses polarity as the tuning passes through an arbitrary value and the other of which is of substantially fixed polarity as said tuning passes through said arbitrary value, and means to combine the two alternating currents and to derive from them a resultant current for controlling the receiver tuning adjustment.

6. The method of signalling with currents keyed between a first frequency and a second frequency and stabilizing the frequency of said currents which includes the following steps, producing, by means of said two currents, pulses which occur when said currents are keyed from said first to said second frequency and vice versa, reversing the polarity of said currents when the frequencies of said two currents deviate with respect to a normal center frequency, producing other pulses when said currents are keyed between said two frequencies, combining said other pulses with said first named pulses, rectifying the combined pulses to produce a current and potential the polarity of which reverses in accordance with reverses in polarity of said first pulses and controlling the frequency of said first currents in accordance with said last named produced potential.

7. The method of signalling with currents keyed between a first frequency and a second frequency and of stabilizing the frequency ofsaid currents during said relaying process which includes the following steps, producing, by means of said two currents, a pair of pulses which occur when said currents shift from said first to said second frequency and vice versa, and the polarity of which reverses when the frequencies of said two currents deviate with respect to a normal center frequenc producing, by means of said currents, a second pair of pulses when said currents deviate from said first to said second frequencies and vice versa, reversing the polarity of one of the pulses of said last named pair of pulses, combining a pulse of each pair with a pulse of the other pair, combining the remaining pulses of said pairs of pulses, rectifying the combined pulses, opposing the rectified currents to produce a current and potential the polarity of which reverses in accordance with reverses in polarity of said first :pulses and controlling the frequency, of said first currents in accordance, with said last named produced potential.

8. In a tunable relay system for currents characteristic of wave energy the frequency of which is shifted between a first frequency and a second frequency, a selective circuit resonant at a frequency intermediate the said two frequencies, connections for impressing said currents on said resonant circuit, a rectifier coupled to said selective circuit, differential rectifiers coupled to said last named rectifier, a demodulator excited by said currents, a, coupling between said demodulator and said differential rectifier system, and a frequency control circuit coupling said differential rectifiers to said tunable relay.

9. In a tunable receiver for receiving frequency shift currents, means for automatically controlling the. tune of said receiver comprising means for deriving one set of current pulses characteristic of shifts in frequency of the received currents, means for deriving another set of current pulses Which correspond to shifts in the frequency of said received currents but which reverse polarity when the receiver adjustment varies above or below a correct value, means to combine and rectify the pulses to provide a current which reverses as the direction of receiver adjustment varies above or below the said cor- -rect value and means excited by said last-named current to correct the adjustment of the tunable receiver. 10. In a tunable relay for relaying currents characteristic of wave energy the frequency of which is shifted between a first frequency and a second frequency, a selective circuit resonant at a frequency intermediate the said two frequencies, connections for impressing said currents on said resonant circuit, a rectifier having an input coupled to said selective circuit and having an output, a pair of transformers each having two primary windings and a secondary winding, a coupling between the output of said rectifier and a primary winding, on each transformer, a pair of rectifiers coupled to the secondary windings of each transformer, connections for adding the outputs of pairs of rectifiers in series, a demodulator excited by said Wave energy for producing pulses when said wave is varied from said first to said second frequency, connections for supplying said last named pulses to the remaining primary windings on said pair of transformers, and a frequency control circuit coupling said connections for adding the outputs of said two pairs of rectifiers in series toa tuner in said tunable relay. i

11. In a signalling system, circuits for receiving wave energy the frequency of which is shifted between a first and second frequency, and apparatus for automatically controlling the tuning of said receiver comprising a demodulator for deriving one set of current pulses characteristic of shifts in frequency of said wave energy between said first and second frequencies, arectifier for deriving. another set of current pulses when said wave energy shifts from said first to said second frequency and which reverse in polarity when the receiver adjustment varies above or below a correct value, connections for combining and rectifying the pulses to provide a current which reverses as the direction of receiver adjustment varies above, or below the saidcorrect value and an automatic frequency control circuit excited by said last-named current to correct the receiver adjustment.

'12. The 'method of detecting variation or drift in the average frequencyof wave energy the frequency of which is alternately keyed from a first frequency through a selected average frequency to a third frequency, and vice versa, which in- ;cludes the following steps, producing from said quency of wave energy the frequency of which is alternately keyed from a first frequency through its average frequency to a third frequency, and vice versa, which includes the following steps,.producing from said wave energy current pulses only in the presence of modulationson said wave energy and simultaneous deviations of the average frequency, causing the polarity of said pulses to reverse when the said average frequency of said wave energy'passes through said selected average frequency, and controlling the average frequency of said wave energy in accordance with said produced pulses.

14. In apparatus for indicating the relation between the frequency of a timing modulated carrier current and the resonant frequency of a circuit, a frequency responsive circuit and a demodulator for deriving modulation frequency currents corresponding to timing modulations of the carrier current, a frequency responsive circuit and a demodulator for deriving modulation frequency currents whichreverse polarity as the carrier current frequency moves through the resonant frequency of said circuit, and a rectifier system'for deriving a direct current from the two modulation frequency currents, the polarity and magnitude of which are-an indication of the frequency of said timing modulated carrier current.

15. In an automatic receiver tuning control system responsive only while the received current is being modulated, a demodulation system for deriving fromthe modulations two alternating currents, one. of which reverses polarity as the tuning passes through an arbitrary value and the other of which is of substantially fixed polarity when said tuning passes through said arbitrary value, and a rectifier system for combining the alternating currents and for deriving therefrom a resultant current for controlling the receiver" tuning adjustment.

16. In a tunable receiver for receiving frequency shift carrier currents, a demodulator for deriving one set of current pulses characteristic of shifts in the frequency of the received currents, a circuit for impressing said currents on said one demodulator, a second demodulator for deriving another set of current pulses which correspond to shifts in the frequency of said received currents but which reverse polarity when the receiver adjustment varies above or below a correct value, a differential rectifier system coupled to both of said demodulators for rectifying the pulses to provide a current which reverses as the directionof receiver adjustment varies above or below the said correct value and'a frequency control circuit excited by said last-named current for correcting the adjustment of the tunable re- YCLARENQE W. HANSELL. 

